Project Summary Breast cancer is a major public health problem with one in eight women in the United States developing breast cancer at some point within her life [1]. Some breast cancer risk factors are related to tissue remodeling during pregnancy and nursing [2]. They include, late age (>35 years) at first full term pregnancy, never becoming pregnant (nulliparity), and never breastfeeding (lactation) [3, 4]. Breast tissue development during pregnancy and lactation is mediated by the actions of progesterone receptor and prolactin receptor, in addition to other hormones [5, 6]. In addition, epigenetic regulation is a likely driver of tissue remodeling that occurs during pregnancy [7]. This study will investigate epigenetic remodeling in the breast during pregnancy with specific interest in the activity of the key pregnancy hormones, progesterone and prolactin. In addition, it will seek to parse out the roles of the two progesterone isoforms, PRA and PRB, in the relationship between parity/lactation and cancer. PRA and PRB which are thought to act in concert to regulate PR-associated gene expression [8, 9]. Aberrant ratios of the isoform composition are found in breast tumors [9], but changes in the ratios have not been documented in the context of the normal process of pregnancy. Thus, understanding progesterone receptor biology in the context of normal breast remodeling during pregnancy is important to the relationship between parity and breast cancer risk. By integrating information about parity induced epigenetic changes with binding events mediated by both isoforms of progesterone receptor and prolactin receptor (or transcription factors downstream of prolactin receptor), a greater understanding of the molecular underpinnings by which parity and lactation alter breast cancer risk will be achieved. This project seeks to elucidate the role of progesterone receptor and prolactin receptor as hormonal drivers of epigenetic reprogramming during parity and lactation, with a focus on the epigenome as a biological mechanism linking early life pregnancy with later life protection against breast cancer development. The aims of the study are: Aim 1: Describe pregnancy induced chromatin modifications to breast epithelium during and after pregnancy and lactation in a mouse model. Aim 2: Identify chromatin structure and gene expression changes in response to progesterone receptor and prolactin receptor activity using an in vitro cell culture model. Aim 3: Characterize the role of isoform biology in the epigenetic and transcriptional response of progesterone receptor in an in vitro cell culture model.